Process for isolating acrylic acid from aqueous crude acrylic acid by extraction with a ketone and plural stage distillation

ABSTRACT

Pure acrylic acid is isolated from an aqueous crude acid containing acrylic acid together with minor proportions of acetic acid, formaldehyde and high boilers boiling at a temperature higher than 220*C by liquid-liquid extraction with the use of 3,3,5-trimethylcyclohexanone and/or isophorone as the extractant(s). To this end, the aqueous acrylic acid-containing crude acid is subjected in an extraction zone to counterflow extraction with the use of the extractant(s) and with the resultant formation of an organic extract containing acrylic acid, acetic acid, high boilers and minor proportions of water, which are dissolved in the extractant; the extract is introduced approximately into the midsection of a first distilling column, which is operated under reduced pressure; the base product of said first distilling column, containing the high boilers and the extractant(s) is recycled to the extraction zone; distillate containing acrylic acid, acetic acid, water and minor proportions of extractant(s) is delivered approximately to the midsection of a second distilling column, which is operated under reduced pressure; a mixture of water and extractant(s) is distilled off; the base product of said second distilling column, containing acrylic acid and acetic acid is introduced into a third distilling column, which is operated under reduced pressure; and acetic acid is separated therein as a distillate and acrylic acid is separated therein as the base product.

United States Patent Sennewald et al.

[451 Sept. 5, 1972 :22] Filed:

[54] PROCESS FOR ISOLATING ACRYLIC ACID FROM AQUEOUS CRUDE ACRYLIC ACIDBY EXTRACTION WITH A KETONE AND PLURAL STAGE DISTILLATION [72]Inventors: Kurt Sennewald, 6 Seinsheim Weg.,

' Hurth-Hermulheim; Heinz Erpenbach, 22 Giesdorfer Strasse, .Surth;

Heinz Handte, 50 Heu'markt,

Cologne; Winiried Lork, l Tulpenweg, Friesheim, all of Germany .Ian. 18,I971 [21] 'Appl. No.: 106,988

[30] Foreign Application Priority Data FOREIGN PATENTS OR APPLICATIONS997,324 7/1965 Great Britain ..260/526 N Primary Examiner Wilbur L.Bascomb, Jr. Attorney-Connolly & Hutz [57] ABSTRACT Pure acrylic acid isisolated from an aqueous crude acid containing acrylic acid togetherwith minor proportions of acetic acid, formaldehyde and high boilers.

boiling at a temperature higher than 220C by liquidliquid extractionwith the use of 3,3,5-trimethylcyclohexanone and/or isophorone as theextractant(s). To this end, the aqueous acrylic acid-containing crudeacid is subjected in an extraction zone to counterflow extraction withthe use of the extractant(s) and with the resultant formation of anorganic extract containing acrylic acid, acetic acid, high boilers andminor proportions of water, which are dissolved in the extractant; theextract is introduced approximately into the midsection of a firstdistilling column, which is operated under reduced pressure; the baseproduct of said first distilling column, containing the high boilers andthe extractant(s) is recycled to the extraction zone; distillatecontaining acrylic acid, acetic acid, water and minor proportions ofextractant(s) is delivered approximately to the midsection of a seconddistilling column, which is operated under reduced pressure; a mixtureof water and extractant(s) is distilled off; the base product of saidsecond distilling column, containing acrylic acid and acetic acid isintroduced into a third distilling column, which is operated underreduced pressure; and acetic acid is separated therein as a distillateand acrylic acid is separated therein as the base product.

7 Claims, 1 Drawing Figure PROCESS FOR ISOLATING ACRYLIC ACID FROM YAQUEOUS CRUDE ACRYLIC ACID BY EXTRACTION WITH A KETONE AND PLURAL STAGEDISTILLATION ing organic extract is distilled and when ethyl acetate isthe extractant an azeotropic mixture of ethyl acetate and water isobtained near the head of the column. The column base product is amixture consisting of 80 weight percent of acrylic acid and 20 weightpercent of acetic acid. The mixture is distilled once again andextensively purified acrylic acid having a strength of 98.6 percent isobtained as the column base product, in a yield of 75 per cent of thetheoretical. In view of the high concentration of acrylic acid in thedistillate, which is composed of 50.6 weight percent of acrylic acid and49.4 weight percent of acetic acid, it is necessary for the distillateto be purified once again, which does not add to the economy of thisprocess. A further disadvantage of thisprocess resides in the highexpenditure of-energy that is occasioned by the need to distil the totalquantity of extractant used, in view of its boiling point which is lowerthan that of acrylic acid.

' A further process has been described in French Pat. No. 1,405,764,wherein pure acrylic acid is isolated from an aqueous crude acid byextracting it preferably with the use of diisopropylether or isopropylacetate as the extractant and simultaneously washing the extract withwater flowing countercurrently thereto. Subjecting the extract tofinishing treatment results in the formation of a concentrate containing97.8 weight percent of acrylic acid and 2.2 weight percent of aceticacid. This concentration of acetic acid in the acrylic acid concentrateindicates that merely about 60 weight percent of the acetic acidinitially present in the crude acid is removed by thisconventional-purification. Acrylic acid having about 2 weight percent ofacetic acid therein is, however, unsuitable for use in either themanufacture of acrylic acid esters, or polymerization reactions. Inother words, this process also fails to satisfy common technicalstandard requirements.

Further processes have been described in British Pat. Nos. 997,325 and1,167,487, wherein pure acrylic acid is isolated from an aqueous crudeacid containing acrylic acid together with minor proportions of aceticacid and possibly formaldehyde by extracting the acid with ethyl acetateand simultaneously washing the resulting organic extract with water.Following this, the washed extract is distilled to isolate firstly pureacrylic acid and secondly a low-boiling azeotrope consisting of ethylacetate and water. Once again, however, it is necessary in theseprocesses to subject the totalquantity of extractant to distillationwith heavy expenditure of energy.

A process similar to that reported in British Pat-No. l,l67,487 has beendescribed in earlier German Pat. No. Application P 18.05.758. 4, savethat the aqueous crude acid having acrylic acid therein is extractedwith the use of 3,3,5-trimethylcyclohexanone or isophorone as a highboiling extractant. The resulting organic extract is washed with waterflowing countercurrently thereto, and dehydrated in a first distillingstage. The base product of the first distilling stage is delivered to asecond'distilling stage and pure acrylic acid is distilled off therein.

The present invention now provides a process which combines a beneficialefiect of the process reported in French Pat. of Addition No. 86,090,namely the omission of the counterflow water wash near the head of theextraction column, with the beneficial effect of the process describedin earlier GermanPatent Application P 18.05.758. 4, namely the use of ahigh-boiling extractant. The quantity of wash water supplied heretoforeto the head of the extraction column-substantially corresponded this inview of the distribution coefficient to the quantity of water alreadypresent in a 25 weight percent solution of crude acrylic acid, forexample. In other words, 6,000 metric tons rather than 3,000 metric tonsof waste water are obtained for a monthly production of 1,000 metrictons of acrylic acid, for example.

The process of the present invention for isolating pure acrylic acidfrom an aqueous crudeacid contain ing acrylic acid together with minorproportions of proportions of water, which are dissolved in theextractant; introducing the extract approximately into the midsection ofa first distilling column, which is operated under reduced pressure;recycling the base product of said first distilling column, containingthe high boilers and the extractant(s) to the extraction zone;delivering distillate containing acrylic acid, acetic acid, water andminor proportions of extractant(s) approximately to the midsection of asecond distilling column, which is operated under reduced pressure;distilling off a mixture of water and extractanfl s); introducing thebase product of said second distilling column, containing acrylic acidand acetic acid into a third distilling column, which is operated underreduced pressure; and effecting separation therein of the acetic acid asa distillate and of the acrylic acid as the base product of said thirddistilling column.

Further preferred features of the present invention comprise:

a. removing from the first distilling column a portion of base productcontaining high boilers and extractant; distilling the said base productportion and merely recycling to the extraction zone extractantdistillatively freed from the high boilers;

b. delivering aqueous raffinate, which is obtained in the extractionzone and contains dissolved extractant,

approximately to the midsection of a stripping column; recovering theextractant by removing an azeotrope of extractant and water overhead;recycling the azeotrope to the extraction zone; and discharging wastewater through the base of the stripping column;

c. combining the mixture of water and extractant, which is obtained asthe distillate of the second distilling column, with the raff'matecoming from the extraction zone, and introducing the combinationapproximately into the midsection of the stripping column;

d. operating the first, second and third distilling columns under apressure of between 5 and 200 mm e. using aqueous crude acid containingbetween 5 and 40 weight percent of acrylic acid, between 0.5 and 8weight percent of acetic acid, between 0.5 and .5 weight percent offormaldehyde and between 0.2 and 5 weight percent of high boilers;

f. effecting the extraction of the aqueous crude acid with the use ofbetween 1 and 10 parts by weight, preferably between 2 and 4 parts byweight of extractant, per part by weight of acrylic acid in the aqueouscrude acid.

The following statements further illustrate the process of the presentinvention.

The condensate of crude acrylic acid to be transformed into pure acrylicacid generally originates from the catalytic gas phase-oxidation ofpropylene to acrylic acid, which has been described, for example, inGerman Pat. No. 1.241.817.

The proportion of acetic acid, which initially appears in the condensateof crude acrylic acid but passes into the extract of acrylicacid/isophorone and/or 3,3,5- trimethylcyclohexanone, during theliquid-liquid extraction of the acrylic acid, is not isolated from thesaid extract by counterflow extraction with water but is distillativelyisolated and obtained in pure form. It is thereby possible considerablyto increase the throughput of material through the simplex extractioncolumn. Following distillative separation of the extract into adistillate comprising water, acetic acid, acrylic acid and some minorextractant(s), and into a base product comprising the bulk ofextractant(s) and the high boilers, the distillate is delivered to acolumn and distillatively freed from water and extractant(s), whilst thebase product is conveyed to a third distilling column and separatedtherein into pure acetic acid and pure acrylic acid.

The wash-water needed heretofore for the re-extraction of acetic acidfrom the extract is economized in the process of the present invention.This means a 50 percent reduction of the costs of waste water disposal,for an acrylic acid production facility. Of prime importance is the factthat the waste water obtained in the present process contains so minorproportions of organic substances that costly biologic wastewater-treatment is rendered unnecessary.

A further advantage of the present process resides in the formation ofconcentrated acetic acid. The process of the present invention will nowbe described with reference to the accompanying flow scheme.

A condensate of crude acrylic acid originating from propylene oxidationis delivered through conduit 1 to the upper portion of a liquid-liquidpulsation and extraction column 2. The pulsating column is packed withstainless steel spirals and has an efficiency corresponding to that ofeight theoretical trays. An extractant mixture of isophorone and3,3,5-trimethylcyclohexanone is introduced into the base of column 2through conduit 3 with the result that the acrylic acid and acetic acidgo into the organic phase and travel upwardly jointly with theextractant. Water droplets which are carried along deposit in a calmingzone near the head of the extraction column and drop back into thecolumn base. The

extract containing acrylic acid, acetic acid, water and high boilers,which are dissolved in the extractant, is

pumped through conduit 4 to the expulsion zone of first distillingcolumn 6. Thermally induced polymerization of acrylic acid is avoided bydistillative isolation of water, acetic acid and'acrylic acid underreduced pressure, preferably under a pressure of between 10 and 200 mmHg, corresponding to a head temperature of between 20 and C. Column 6may be a packed column or bubble tray column with between 10 and 20theoretical trays in its expulsion zone, and the same number of trays inits reflux intensifier zone.

The head product coming from column 6 and consisting of acrylic acid,acetic acid, water and a-quantity of extractant, corresponding to theazeotropic mixture with water, is delivered through conduit 8 to seconddistilling column 9, which may also be a packed column or bubble traycolumn. Column 9 is operated under reduced pressure of between 10 and100 mm Hg and the base product is disintegrated therein into firstly anazeotropic water-extractant mixture as the distillate,

and secondly acrylic acid and acetic acid as the base product. Thedistillate coming from column 9 travels through conduit 11 and isintroduced into conduit 5 described hereinafter, in which residualextractant is recovered. The base product coming from column 9 isconveyed through conduit 10 to a third distilling column 12, in whichacetic acid is obtained as a distillate conduit 14 and acrylic acid isobtained as a base product conduit 13. The base product of column 12,which consists of pure acrylic acid and some minor proportions ofpolymers, may be evaporated in a thin film evaporator (not shown in thedrawing), and pure acrylic acid is obtained therein as the distillate.The minor polymer proportions are retained therein as nonvaporizableresidue. Both column 12 and the thin film evaporator are operated undera preferred pressure of between 10 and 200 mm Hg.

The base product of column 6, which consists of extractant and highboilers that are dissolved therein, is removed through conduit 7 andrepumped to extraction column 2, through conduits 17 and 3. Accumulationof high boilers is obviated by withdrawing a portion of extractantmixture from the cycle, through conduit 18, freeing it fromundistillable residues in a thin film evaporator (not shown in thedrawing) and reintroducing it into the extractant cycle.

Water (raffinate) obtained in the bottom portion of extracting column 2is delivered through conduit 5 to column 15 in which the extractantdissolved in the water is stripped off and recovered. Column 15 isoperated at atmospheric pressure. The dissolved extractant is recoveredazeotropically as upper phase near the head of the column and deliveredthrough conduits 17 and 3 to extraction column 2, whereas the loweraqueous-phase is returned to column 15. Waste water, which is free fromextractant and contains merely minor proportions of organic substances(formaldehyde) is discharged at the bottom of column 15, through conduit16.

The polymerization of acrylic acid during the distillation of theextract is avoided, preferably by adding a conventional stabilizer, suchas hydroquinone, p-tert. butylpyrocatechol, p-benzoquinone or methyleneblue. The acrylic acid is further stabilized by the addition of acorresponding stabilizer to the distillate and reflux matter coming fromthe overall columns.

The process described hereinabove distinguishes very favorably fromthose used heretofore for the production of pure acrylic acid. This inview of the fact that merely a simplex extraction column with highthroughput rates is required to be used for the extraction of acrylicacid from the aqueous crude acid. The considerably reduced formation ofwaste water and the fact that the waste water need not be subjected tobiologic treatment are further commercially important factors in whichthe present process distinguishes so favorably over conventional andpartially common processes.

EXAMPLE:

Nine, hundred fifty kg of an aqueous condensate of crude acrylic acidcomposed of:

26.5 weight percent of acrylic acid,

1.8 weight percent of acetic acid,

0.8 weight percent of formaldehyde,

1.9 weight percent of high boilers (bp 220C),

0.1 weight percent of hydroquinone, the balance being water, wasintroduced into the upper portion of pulsating extraction column 2. Thecolumn had an effective height of 3 meters and was packed with stainlesssteel spirals (8 mm). 788.8 kg of an extractant mixture composed of 30parts by weight of isophorone and 70 parts by weight of3,3,5-trimethylcyclohexanone was introduced into the base portion ofcolumn 2 and travelled upwardly therein, countercurrently with respectto the condensate. After intimate exchange of material between the twophases, there were obtained, near the head of extraction column 2, 1157.0 kg of an extract composed of:

21.8 weight percent of acrylic acid,

1 .4 weight percent of acetic acid,

1.5 weight percent of high boilers,

7.7 weight percent of water,

0. 1 weight percent of hydroquinone, the balance being extractant.

The extract so produced was supplied to first distilling column 6, whichwas operated under a pressure of 40 mm Hg. Column 6 was subdivided intoan expulsion zone 1.2 meter high and a reflux intensifier zone likewise1.2 meter high, which were both packed with stainless steel spirals 6mm). The head product obtained at a head temperature of 58C consisted of368.0 kg of a distillate composed of:

4.5 weight percent of acetic acid,

68.2 weight percent of acrylic acid,

3. 1 weight percent of extractant, and

24.2 weight percent of water.

789 kg of a base product composed of:

0.2 weight percent of acrylic acid,

2.3 weight percent of high boilers,

, the balance being extractant, was discharged through the bottom ofcolumn 6.

The base product was supplied to a thin film evaporator and freedtherein from 17.9 kg of high boilers, whereas the distillate coming fromthe thin film evaporator was recycled to extraction column 2.

The distillate coming from column 6 was introduced into distillingcolumn 9, which was operated under a pressure of mm Hg and in which itwas separated into 100.2 kg of a distillate composed of 89.0 weightpercent of water and 1 1.0 weight percent of extractant, and 267 kg of abase product. The distillate obtained in column 9 was delivered throughconduits 11 and 5.to stripping column 15, in which residual extractantwas stripped off and recovered.

The base product of column 9 was supplied to a packed column 12, whichwas operated under a pressure of 40 mm Hg and subdivided into a refluxintensifier zone 1 m high and an expulsion zone 1.5 m high, and the baseproduct was separated therein into 16.3 kg of a head product and 251.5kg of a base product. The head product coming from column 12 wascomposedof:

98.2 weight percent of acetic acid,

0.6 weight percent of acrylic acid,

0.6 weight percent of water, and

0.6 weight percent of extractant.

The base product of column 12 was pure acrylic acid which contained 0.4weight percent of polymers and 0.2 weight percent of acetic acid.

The aqueous rafl'inate coming from extraction column 2 was combined withthe distillate coming from column 9. This gave a total quantity of 681.8kg which contained minor proportions of acetic acid, formaldehyde andhigh boilers together with 2.6 weight percent of extractant. Thisquantity was introduced into stripping column 15, wherein the extractantwas stripped off and recovered. Column 15 was packed with Raschig rings(6 mm) and operated at atmospheric pressure. The extractant recoveredtherein (17.4 kg) was combined with fresh extractant flowing throughconduit 3, and the extractant combination was used again in extractioncolumn 2.

664.4 kg of waste water composed of:

0.1 weight percent of acrylic acid,

0.1 weight percent of acetic acid,

1.1 weight percent of formaldehyde,

0.2 weight percent of high boilers,

The balance being water, was discharged through the bottom of strippingcolumn 15.

Substantially 99 percent of the crude acrylic acid used were recoveredas pure acrylic acid. The balance to 100 was lost together with thewaste water or transformed into high-boiling polymers.

We claim:

1. A process for isolating pure acrylic acid from an aqueous crude acidcontaining acrylic acid together with minor proportions of acetic acid,formaldehyde and high boilers boiling at a temperature higher than 220C,by liquid1iquid extraction with the use of 3,3 ,5-trimethylcyclohexanone, isophorone or mixtures thereof as an extractant,which comprises subjecting the aqueous acrylic acid-containing crudeacid in an extraction zone to counterflow extraction with the use of theextractant and with the resultant formation of an organic extractcontaining acrylic acid, acetic acid, high boilers and minor proportionsof water dissolved in the extractant; introducingthe extractapproximately into the midsection of a first distilling zone, which isoperated under reduced pressure; recycling the base product of saidfirst distilling column, containing the high boilers and the extractantto the extraction zone; delivering distillate containing acrylic acid,acetic acid, water and minor proportions of extractant approximately tothe midsection of a second distilling zone, which is operated underreduced pressure; distilling off a mixture of water and extractant;introducing the base product of said second distilling column,containing acrylic acid and acetic acid into a third distilling zone,which is operated under reduced pressure; and effecting separationtherein of the acetic acid as a distillate and of the acrylic acid asthe base product of said third distilling column.

2.. The process as claimed in claim 1, wherein a portion of the baseproduct of the first distilling zone, containing the high boilers andextractant, is removed from the said first distilling zone, distilledand the extractant distillatively freed from the high boilers isrecycled to the extraction zone.

3. The process as claimed in claim 1, wherein aqueous raffinate, whichis obtained in the extraction zone and contains dissolved extractant, isdelivered approximately to the midsection of a stripping zone, theextractant is recovered by removing an azeotrope of extractant and wateroverhead, the azeotrope is recycled to the extraction zone, and wastewater is discharged through the base of the stripping zone.

4. The process as claimed in claim. 1, wherein the mixture of water andextractant, which is obtained as the distillate of the second distillingzone, is combined with the rafiinate coming from the extraction zone,and the combination is introduced approximately into the midsection ofthe stripping zone.

5. The process as claimed in claim 1, wherein the first, second andthird distilling zones are operated under a pressure of between 5 and200 mm Hg.

6. The process as claimed in claim 1, wherein the aqueous crude acidcontains between S and 40 weight percent acrylic acid, between 0.5 and 8weight percent acetic acid, between 0.5 and 5 weight percentformaldehyde and between 0.2 and 5 weight percent high boilers.

7. The process as claimed in claim 1, wherein the aqueous crude acid isextracted with the use of between 1 and 10 parts by weight extractant,per part by weight acrylic acid in the aqueous crude acid.

2. The process as claimed in claim 1, wherein a portion of the baseproduct of the first distilling zone, containing the high boilers andextractant, is removed from the said first distilling zone, distilledand the extractant distillatively freed from the high boilers isrecycled to the extraction zone.
 3. The process as claimed in claim 1,wherein aqueous raffinate, which is obtained in the extraction zone andcontaiNs dissolved extractant, is delivered approximately to themidsection of a stripping zone, the extractant is recovered by removingan azeotrope of extractant and water overhead, the azeotrope is recycledto the extraction zone, and waste water is discharged through the baseof the stripping zone.
 4. The process as claimed in claim 1, wherein themixture of water and extractant, which is obtained as the distillate ofthe second distilling zone, is combined with the raffinate coming fromthe extraction zone, and the combination is introduced approximatelyinto the midsection of the stripping zone.
 5. The process as claimed inclaim 1, wherein the first, second and third distilling zones areoperated under a pressure of between 5 and 200 mm Hg.
 6. The process asclaimed in claim 1, wherein the aqueous crude acid contains between 5and 40 weight percent acrylic acid, between 0.5 and 8 weight percentacetic acid, between 0.5 and 5 weight percent formaldehyde and between0.2 and 5 weight percent high boilers.
 7. The process as claimed inclaim 1, wherein the aqueous crude acid is extracted with the use ofbetween 1 and 10 parts by weight extractant, per part by weight acrylicacid in the aqueous crude acid.